1. Field of the Invention
This invention relates generally to mechanical seals and, more specifically to cartridge mounted mechanical seals which circulate barrier fluid for cooling and lubrication purposes.
2. Prior Art
Mechanical seals are used to provide a seal between various components on a rotating shaft. Such seals take many different forms. In the field of cartridge mounted seals are mechanical seals which have at least one set of contacting seal faces, one rotary face and one stationary face, the rotary face being driven by its connection to a sleeve which is mounted nonrotatably to the shaft. The entire cartridge seal unit is installed by simply sliding the sleeve and the attached seal components onto the shaft and fixing the sleeve in place (usually with set screws). A gland plate is provided for supporting the stationary seal face. The gland plate is usually bolted to the housing (such as a pump housing) through which the shaft extends. If two sets of seal faces are provided on either side of the gland plate, the seal is known as a double or tandem seal.
In double or tandem seals, and in some specialized applications of single cartridge mounted seals, a barrier fluid is circulated through the seal during operation. Barrier fluid, such as a water-ethylene glycol mix, enters and exits through the gland plate. The fluid circulating within the seal provides lubrication for the sealing faces as well as coolant for the seal. The barrier fluid also can help absorb vapor leakage of process fluids, protecting the environment. In a double seal the pressure is greater in the seal than in the process stream. In a tandem seal the pressure of the process stream is greater than in the seal. Due to space limitations, maintenance problems and other site specific requirements, a separate device (such as a pump) for circulating the fluid through the seal is not desirable. Prior art means for providing barrier fluid include simply connecting the inlet and outlet to a barrier fluid reservoir, allowing convection currents to provide some circulation for the fluid. In such systems the seals may experience early failure due to lack of lubrication and extensive heat damage. Another system utilized to circulate barrier fluid involves flowing the barrier fluid into the seal cavity and then expelling it to a drain. This process is wasteful of barrier fluid and also presents environmental disposal problems.
Various devices have been designed to utilize the rotation of the shaft to induce a pumping action from within the seal to circulate the barrier fluid. In some mechanical seals an impeller has been added to the interior design of the seal for circulation of barrier fluid. The addition of an impeller is not possible in many cartridge designs as it results in too much bulk in the seal, increasing the overall size of the seal and prohibiting installation of the device in many applications requiring very small seal components.
U.S. Pat. Nos. 4,466,619 and 4,560,173 disclose other attempts to provide a circulation means without adding and impeller-type device to the shaft or the shaft sleeve. U.S. Pat. No. 4,466,619, invented by William V. Adams, describes a pumping device which includes inlet and outlet passages bored through the seal stator and oriented tangentially to the shaft sleeve in order to encourage liquid flow. A pumping rotor is formed on the sleeve by cutting radial rectangular grooves in the sleeve to aid in moving the barrier fluid through the seal. The use of tangential bores often requires a relocation of the inlet and outlet points on the gland. Such relocation may prohibit installation of the seal in applications where there is limited access to the gland for attachment of circulation piping. A large number of installation sites have access to the gland limited to the top and bottom of the seal. Also, tangential boring of the stator and/or the gland is adds expense to the seal manufacturing process. Further, the use of rectangular pumping grooves greatly weakens the shaft sleeve, which has a very thin cross section in many seals. In order to combat the reduced sleeve strength it is necessary to thicken the sleeve cross section, again adding unwanted bulk to the overall seal construction, making installation difficult or impossible in many situations. The location of the pumping grooves directly beneath the stator in the center of the seal does not encourage circulation of barrier fluid to all of the interior seal components, reducing the effectiveness of the device.
U.S. Pat. No. 4,560,173, invented by William V. Adams and Duane A. Avard, attempts to further advance the art of barrier fluid circulation. This patent eliminates the pumping grooves from the shaft sleeve and utilizes an obstruction extending inward from the gland between the inlet and outlet to encourage fluid flow, induced by the rotating shaft. While this improvement eliminates the rectangular pumping grooves, other problems are created. Many cartridge mounted seals utilize an annular flange extending inward from the gland on which to mount various seal components, which are connected to the seal stator. It is preferred in many applications to provide springs or other resilient means between the flange and the stator to allow the stator to adjust to misalignment of the seal and to provide maximum contact between the stator(s) and rotor(s). Since the obstruction of Adams et al. does not extend fully around the shaft sleeve there is no place to install springs or other seal components. Adams et al. is thus limited to designs in which the rotor is resiliently mounted rather than the stator. Such seals do not perform the sealing function when misaligned as well as seals which incorporate self-adjusting stators.
Both of the patented devices described above have also exhibited a limited ability to adequately circulate the barrier fluid. There is an increased concern by those in the mechanical seal industry in circulating barrier fluid at a greater flow rate to all internal seal components, thus making the seals function more efficiently and for a longer period of time.